Technical Field
The present invention relates to a calibration device for a measuring probe, to a coordinate measuring system equipped therewith, and to a measurement method for the coordinate measuring system.
Discussion
Calibration devices for tactile measuring probes, as well as the measuring probes themselves, are known. In machine tools, the use of measuring probes serves, for example, to accurately determine or measure the position, and optionally also the orientation, of workpieces clamped on the tool table. This ensures that the machine tool control, for example of a CNC machine, on the one hand processes, for example mills, clamped workpieces accurately according to specification, and on the other hand prevents the tool, for example the milling head, from being inadvertently driven into the workpiece and thereby damaging the machine tool. The tactile measuring probes for the position determination are generally installed in machine tools only when required, for example in order to measure the clamped workpiece on the working table of the machine tool. The measuring probe is in this case part of a coordinate measuring system which is generally connected to the machine control of the machine tool and communicates measurement points detected by the measuring probe to the machine tool. To this end, in the normal case the measuring probe is clamped at its base in the chuck of the mobile machine head, i.e. instead of a workpiece. This ensures that the measurements are carried out in direct relation to the tool head of the machine tool. At its other end, the tactile measuring probe has a deflectable sensing element, which is usually configured in the shape of a sphere or spherical cap. The sensing element has an accurately defined sensing surface which, for a position measurement, needs to come in contact—pointwise—with the object to be measured. So that damage-free contact of the sensing element on the measuring probe with the object to be measured can be performed at all, the measuring probe is equipped with an electromechanical sensor. This sensor operates like a small spring element. In the event of contact of the deflectable sensing element on the tip of the measuring probe with an object, the measuring probe is compressed—resiliently and reversibly—along its longitudinal axis by a sensing deflection I. Once this sensing deflection reaches a determined threshold value ITr (trigger value, for example in the range of 4-30 μm) then the measuring probe, or its sensor, triggers an electrical signal which signals contact, or compression, of the measuring probe and, for example, prevents further movement of the machine head. Since the geometry of the measuring probe—in particular the geometry of the sensing surface on its sensing element and the length of the measuring probe—are accurately defined and known after calibration on the machine tool, the relative position of a measured point—for example on the clamped workpiece—can be accurately determined, and for example detected by the machine control of the machine tool, on the basis of the position, known to the machine control, of the tool head or of the chuck. Since machine tools are now highly precise, the sensing deflection ITr of the measuring probe also needs to be taken into account in this position determination. This is done during the calibration of the tactile measuring probe.
For an accurate position measurement with a measuring probe, for example, Document WO 98/57121 proposes to record the position of the sensing element on the measuring probe directly with an optical sensor, so that a sensing deflection or otherwise occurring deformations of the generally pin-shaped measuring probe cannot interfere with the position measurement.
Such a device, however, has not always proven reliable in practice because in certain cases—for example in the event of contamination—the optical sensor cannot in fact detect the sensing element correctly. The device is furthermore relatively complex because it is necessary to move the measuring probe and the additional optical sensor with the measuring probe. Furthermore, there are applications in which the sensing element cannot be kept in the region of view of the optical sensor during a measurement. If the intention is to measure the depth of a bore, for example, the optical sensor cannot detect the sensing element inserted into the bore, and consequently cannot carry out a measurement. The usability of such a measuring device is therefore relatively limited.
WO 94/08205 describes a similar device, in which the mechanical sensing element is moved by means of a video camera to the position to be measured.
A further calibration device for mobile devices is described in DE 10027106-A1.
EP 2203273-B1 in the name of the company Conoptica discloses a measuring device with which rotating tools can be measured. In its paragraphs [105] to [107], however, the document here also describes the possibility of measuring the above-described tactile measuring probes with the same measuring device. To this end, measuring probes are clamped in the chuck of the tool head and measured in a measuring device, positioned for example next to the machine table (see paragraph [106] and FIGS. 6a and 7a). To this end, the measuring device has an optical detector and a pneumatic rod, on which a glass cube having a reference pattern is fastened. The glass cube is initially brought into the field of view of the optical detector. The detector can detect the precise position of the glass cube, or of its surface, with the aid of the reference pattern applied on the glass cube. Subsequently, the tool head moves the tip of the measuring probe against the glass cube until the tactile measuring probe signals contact and stops further movement of the machine head. On the basis of the known position of the base of the measuring probe in the chuck and of the touched reference surface on the glass cube, the measuring device can determine both the length of the measuring probe in the triggered or touched state and the actual sensing deflection lTr of the measuring probe, and therefore calibrate the measuring probe.
If the measuring probe is fitted in the tool head again at a later time, then the calibration can be carried out more rapidly. To this end, the spherical tip of the measuring probe is brought into the field of view of the optical detector again, and the position or the coordinates of the spherical tip are determined optically and compensated with the stored sensing deflection ITr of the measuring probe. This gives the effective position which the spherical tip occupies in the triggered state, i.e. the state of touching a measurement point. The newly fitted measuring probe is therefore already calibrated and ready for new position measurements.
Although the measuring device of EP 2203273-B1 gives good measurement results, the use of the glass cube for the sensing deflection measurement ITr is very elaborate in practical use and consequently needs improvement since, for the sensing deflection measurement ITr, the glass cube additionally needs to be provided with a transparent glass platelet and frozen (in order to produce the reference pattern, see the end of paragraph [107]). Furthermore, the glass cube and its reference surface (which touches the measuring probe) need to be produced with high precision so that it allows a very precise measurement. The design is consequently relatively complex and therefore expensive.